This invention relates to a measuring cell for an Ion Cyclotron Resonance (ICR) mass spectrometer.
Fourier Transform Ion Cyclotron Resonance (FT-ICR) is a technique for high resolution mass spectrometry.
Ion motion in a homogeneous magnetic field in a plane perpendicular to the direction of the field represents a circular orbit. This circular orbiting of an ion is termed “cyclotron motion” or “cyclotron oscillation”. The frequency of the cyclotron oscillation is inversely proportional to the mass-to-charge ratio m/z of the ion and directly proportional to the strength of the magnetic field. This motion is usually measured by a detection of the image current induced by an ensemble of oscillating ions on an electrode (called “detection electrode”) followed by a subsequent Fourier transformation of the signal. This gives a spectrum of the frequencies of the cyclotron oscillations of simultaneously trapped ion ensembles and hence ionic mass-to-charge ratios m/z, which serves as a basis for the FT-ICR mass spectrometry method.
In order to constrain ion motion in the direction along the homogeneous magnetic field and to detect ion motion, FT-ICR mass spectrometers confine ions in cells (sometimes called traps) of various configurations. Descriptions of a number of FT-ICR cells can be found for example in the publication of Shenheng Guan, and Alan G. Marshall; International Journal of Mass Spectrometry and Ion Processes 146/147 (1995) 261-296. The ion motion of the ions trapped inside the cell is restrained in the plane perpendicular to the magnetic field by the magnetic field itself, and in the dimension along the magnetic field by an electrostatic trapping potential. The ion motion inside the cell can generally be represented as a superposition of three periodic motions:    (1) an oscillation along the axis z parallel to the magnetic field called trapping oscillation,    (2) a cyclotron rotation in the plane perpendicular to the magnetic field, and    (3) a magnetron drift motion in that plane generated by radial electrostatic forces.The frequencies of these motions are usually denoted as ωz, ωc and ωm respectively.
For generating mass spectra, a “reduced ion cyclotron frequency ω+” is measured which is composed of the above frequencies of motions. The true cyclotron frequency ωc cannot be measured directly. The reduced cyclotron frequency can be calculated theoretically from the cyclotron frequency ωc and the trapping frequency ωz. As long as the electrostatic trapping potential is quadrupolar, the reduced cyclotron frequency ω+ does not depend on the axial and radial positions of the ion inside the cell, and a high mass resolution is achieved. The quadrupolar potential is produced by cell electrodes formed as hyperbolic surfaces. A trapping potential more or less approximating the ideal quadrupolar one exists in the vicinity of the center of a cell of any geometry, particularly in cylindrical cells. The size of the region with sufficiently good quadrupolar trapping potential depends on the form of the cell.
The electrodes to which the trapping potential is applied are called trapping electrodes. The trapping electrodes usually arranged essentially perpendicular to the direction of the magnetic field.
The frequency ω+ of ion motion is usually detected via an image charge induced on cell electrodes called detecting electrodes. The detecting electrodes usually are lengthy electrodes essentially parallel to the magnetic field lines. In conventional FT-ICR cells, the detection signal increases when the diameter of the cyclotron motion becomes larger, and when ions of the same mass-to-charge ratios are moving in the same phase. This is valid up to the point where the ion orbit becomes comparable with the internal dimension of the cell, i.e. when the ions fly near to the detection electrodes. To obtain such a coherent motion with an enhanced cyclotron radius, the cyclotron oscillations of trapped ions are usually excited by subjecting them to an oscillating electric field applied perpendicular to the direction of the magnetic field and having a frequency equal to the cyclotron frequency of the ions. This excitation electric field is applied to so-called excitation electrodes of the cell. Sometimes the same electrodes are used for both excitation and detection, but it is more common to have separate excitation and detection electrodes.
The excitation/detection and trapping electrodes must not be plane electrodes. They may have the surface of a cube, or cylinder, or hyperboloid of revolution. According to the shape of the surface the cell is then referred to as cubical or cylindrical of hyperbolic cell, respectively.
The main disadvantage of the currently used FT-ICR cell designs is the long acquisition time required to achieve good resolving power. Because of the principle limitations associated with the Fourier transform, the signal acquisition duration T to obtain resolution R is given byT=4πR/ωc  (1)[Jonathan Amster; Journal of Mass Spectrometry, vol. 31, 1325-1337 (1996)].
Thus short analyses times result in low resolution. To overcome this limitation, it was suggested to use multielectrode detection plate arrangements [E. N. Nikolaev et al. SU patent 1307492 A1 (1985). Alan Rockwood et al., U.S. Pat. No. 4,990,775 (1991)]. In these arrangements, each of the detection electrodes is split into several smaller electrodes, and they are connected to an amplifier of the image signal in such a way that the detection occurs on a multiple of the reduced cyclotron frequency ω+, e.g on n·w+, where n is integer.
The main drawback of the multiple electrode detection cells is their low sensitivity. This drawback results from the fact that an ion residing inside an FT-ICR cell induces an image signal on all cell electrodes simultaneously. Since only some of the electrodes are used for detection, the detection efficiency is reduced compared to a cell entirely consisting of detecting electrodes. Furthermore, for efficient detection some of the detecting electrodes should be connected to a positive pole of an image signal amplifier, while other detection electrodes should be connected to the negative pole of the same amplifier, and during the detection an ion must come close to detecting electrodes of different polarity in alternating order. It is essential, that an ion induces at a given time an image signal preferentially in one of the detector plates only. This is achieved for diameters of the cyclotron orbits close to the cell dimension in the plane of the cyclotron motion. To obtain the same sensitivity with a multielectrode cell, the cyclotron diameter has to be larger for larger n.
However, diameters exceeding approximately half of the cell dimension lead to an increase in the amplitudes of parasitic harmonics, i.e. undesired signals occurring on the frequency m·w+. The desire to limit the amplitude of higher harmonics, in practice to below approximately 10% of the total signal for each harmonic frequency, requires a limitation of the excitation of the ion's cyclotron orbits to diameters smaller than half of the cell dimension, which leads to a low sensitivity of the detection, especially for multielectrode cells. Another reason for keeping diameters of the ion cyclotron motion relatively small is that for all cells (except for the ideal hyperbolic cells) the trapping potential deviates from the quadrupolar one for relatively large distances from the center of the cell. This deviation leads to the change in ω+ for ions excited to different cyclotron orbits and thus in the degradation of resolution and mass accuracy.
Therefore, there is a need to keep radii of the ion cyclotron motion small compared to the inner radius of the cell in a plane perpendicular to the direction of the magnetic field. But this requirement leads to the decrease in sensitivity of the measurements in all prior art cells.